Method for preparing a hydro(chloro)fluoroalkane and catalyst

ABSTRACT

Process for the preparation of a hydro(chloro)fluoroalkane according to which a halogenated precursor of the hydro(chloro)fluoroalkane is reacted with hydrogen fluoride in the presence of a catalyst comprising chromium (Cr) and at least one other metal selected from the group consisting of aluminium, barium, bismuth, calcium, cerium, copper, iron, magnesium, strontium, vanadium and zirconium.

[0001] The present invention relates to a process for the preparation ofa hydro(chloro)fluoroalkane and a catalyst.

[0002] When a halogenated precursor of a hydro(chloro)fluoroalkane isreacted in the presence of hydrogen fluoride under fluorinationconditions, it is difficult to selectively obtain a desiredhydro(chloro)fluoroalkane. The formation of other reaction products,which often cannot be recycled or used, such as undesired isomers ordismutation products, reduces the productive output ofhydro(chloro)fluoroalkane.

[0003] It is, for example, known from JP-A02/178237 to carry out ahydrofluorination of perchloroethylene with hydrogen fluoride in thepresence of a catalyst comprising chromium, magnesium and aluminiumoxides. This process has an unsatisfactory productive output ofpentafluoroethane (HFC-125). The process is not more satisfactory withregard to the selectivity for 1,1,1-trifluoro-2,2-dichloroethane(HCFC-123).

[0004] It was therefore desirable to find a process which would makepossible access in a controlled and selective manner to a specifichydro(chloro)-fluoroalkane.

[0005] The invention consequently relates to a process for thepreparation of a hydro(chloro)fluoroalkane according to which ahalogenated precursor of the hydro(chloro)fluoroalkane is reacted withhydrogen fluoride in the presence of a catalyst comprising chromium (Cr)and at least one other metal (M) selected from aluminium, barium,bismuth, calcium, cerium, copper, iron, magnesium, strontium, vanadiumand zirconium, in which process the catalyst is depleted in ammoniumions.

[0006] It has been found, surprisingly, that the process according tothe invention makes possible access with high selectivity to a specifichydro(chloro)-fluoroalkane with high overall selectivity forhydro(chloro)fluoroalkane and for hydrochlorofluorinated precursors ofthe said hydro(chloro)fluoroalkane.

[0007] In the process according to the invention, the catalyst typicallyincludes at most 1% by weight of ammonium ions. It preferably exhibits acontent of ammonium ions of at most 0.5% by weight. The content ofammonium ions in the catalyst is preferably at most 0.2% by weight.Excellent results are obtained with a catalyst for which the content ofammonium ions is at most 0.1% by weight. Particularly advantageousresults are obtained with a catalyst for which the content of ammoniumions is at most 0.05% by weight.

[0008] The catalyst can be a bulk or supported catalyst. A bulk catalystis preferred.

[0009] The M/Cr atomic ratio of the metal M to the chromium in thecatalyst is generally at least 0.01. The atomic ratio is preferably atleast 0.05, advantageously at least 0.1. The atomic ratio is generallyat most 100. It is most often at most 20. It is preferably at most 10.

[0010] The catalyst generally exhibits a specific surface determinedaccording to the BET method with nitrogen, of at least 15 m²/g,preferably of at least 25 m²/g. The specific surface is generally atmost 200 m²/g. It is preferably at most 100 m²/g.

[0011] In the process according to the invention, the catalyst ispreferably obtained by fluorination of a mixed oxide of chromium and ofmetal M. The fluorination is preferably carried out with hydrogenfluoride, optionally diluted with an inert gas, such as nitrogen orhelium. The duration of the fluorination is generally from 1 to 100 h.The fluorination temperature is generally between 150 and 400° C. It ispreferably at most 350° C. The fluorination can be carried out, forexample, immediately before the reaction of the halogenated precursorwith hydrogen fluoride, preferably in the reactor used for the latterreaction.

[0012] The preparation of the mixed oxide preferably comprises a stageof coprecipitation by reaction of an aqueous solution of soluble metaland chromium salts, which is reacted an aqueous ammonia solution

[0013] The preparation of the mixed oxide can advantageously compriseone or more drying or calcination stages. The temperature of thecalcination is generally from 150° C. to 400° C. The calcinationtemperature is preferably at most 350° C. A calcination temperature ofat most 340° C. is more particularly preferred. The calcination is oftencarried out at a temperature of at least 200° C.

[0014] After calcination, the mixed oxide generally exhibits a specificsurface, determined according to the BET method with nitrogen, of atleast 150 m²/g, preferably of at least 180 m²/g. The specific surface ofthe mixed oxide is generally at most 450 m²/g.

[0015] The preparation of the catalyst generally comprises a treatmentintended to reduce, preferably to essentially completely remove, thecontent of ammonium ions in the catalyst. This treatment can, forexample, be at least one washing, preferably with water, or a heattreatment or a combination of these treatments. The effectiveness of thetreatment intended to reduce the content of ammonium ions is generallyconfirmed by methods known per se. For example, Nessler's reagent can beused to analyse the content of ammonium ions. This treatment ispreferably applied to the mixed oxide precursor of the catalyst, beforesubjecting it to the fluorination stage.

[0016] In a particularly preferred alternative form, the preparation ofthe catalyst comprises the following stages:

[0017] (a) a preparation of a mixed oxide of metal M and of chromium bycoprecipitation from an aqueous solution of soluble metal and chromiumsalts, which is reacted with an aqueous ammonia solution;

[0018] (b) a washing of the mixed oxide, intended to reduce, preferablyto essentially completely remove, the content of ammonium ions in thecatalyst;

[0019] (c) a calcination of the washed mixed oxide;

[0020] (d) a fluorination treatment of the calcined mixed oxide.

[0021] The catalyst and the mixed oxide are generally essentiallyamorphous. They are preferably completely amorphous.

[0022] In the process according to the invention, the reaction betweenthe hydrogen fluoride and the chlorinated precursor is usually carriedout at a temperature of 150 to 450° C. The reaction is preferablycarried out in the gas phase. The pressure of the reaction is usuallyfrom 0.5 to 30 bar. The molar ratio of the hydrogen fluoride to thehalogen precursor is usually from 1 to 100. The residence time isusually from 1 to 1000 s.

[0023] Halogenated precursors which can be used in the process accordingto the invention are known. The halogenated precursor is preferably achlorinated precursor. Mention may be made, by way of example, ofdichloromethane, trichloromethane, trichloroethylene, perchloroethyleneand pentachloroethane.

[0024] In an alternative form, the process according to the inventionapplies to the synthesis of a hydrochlorofluoroalkane. In thisalternative form, the metal M is advantageously selected from barium,bismuth, copper, iron, magnesium and strontium. The metal is preferablyselected from barium, bismuth, magnesium and strontium. This alternativeform of the process according to the invention has proved to beadvantageous for preparing chlorofluoromethane, chlorodifluoromethane,2-chloro-1,1,1-trifluoroethane or 2,2-dichloro-1,1,1-trifluoroethane.

[0025] This alternative form has proved to be particularly advantageousfor preparing 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123). In thiscase, the halogenated precursor advantageously comprises at least onechlorinated organic compound selected from perchloroethylene,1,1,2-trichloro-2-fluoroethylene, 1,1,2-trichloro-2,2-difluoroethane and1,1,2,2-tetrachloro-2-fluoroethane. The chlorinated organic conpound ispreferably selected from 1,1,2-trichloro-2,2-difluoroethane andperchloroethylene.

[0026] In another alternative form, the process according to theinvention applies to the synthesis of a hydrofluoroalkane. In thisalternative form, the metal is advantageously selected from aluminum,calcium, cerium, vanadium and zirconium. Zirconium and aluminum arepreferred. Aluminium is particularly preferred.

[0027] In order to reduce the cost of the catalyst in the processaccording to the invention, it is advantageous to use a catalyst with ahigh content of aluminium with respect to the more expensive chromium.It has been found that, in the process according to the invention,hydrofluoroalkanes can be synthesized in a particularly selective andeconomic manner when use is made of a catalyst comprising chromium andaluminium in an Al/Cr atomic ratio of at least 0.5. This ratio ispreferably at least 1. In a particularly preferred way, this ratio is atleast 2. The Al/Cr atomic ratio is advantageously at most 20. This ratiois preferably at most 10. In a particularly preferred way, it is at most5.

[0028] This second alternative form has proved to be advantageous forpreparing difluoromethane, 1,1,2-tetrafluoroethane or pentafluoroethane.

[0029] This alternative form has proved to be particularly advantageousfor preparing pentafluoroethane. In this case, the halogenated precursoris advantageously perchloroethylene or2,2-dichloro-1,1,1-trifluoroethane. The pressure in this case isadvantageously at most 15 bar, preferably less than 10 bar. The pressureis advantageously at least 1 bar. A lower pressure makes possible anadditional increase in the selectivity for pentafluoroethane whereas ahigher pressure makes it possible to increase the overall productiveoutput of the process.

[0030] The invention also relates to the mixed oxide and to the catalystwhich are described above.

[0031] The examples given below are intended to illustrate the inventionwithout, however, limiting it. In the examples, the degree of conversionof the halogenated precursor is the ratio, expressed as per cent, of, onthe one hand, the amount employed minus the unconverted amount to, onthe other had, the amount employed; the selectivity forhydro(chloro)fluoroalkane is the ratio, expessed as per cent, of theamount of hydro(chloro)fluoroalkane formed to the amount which wouldhave been formed if all the halogenated precursor converted hadgenerated hydro(chloro)fluoroalkane; the overall selectivity is the sumof the selectivity for the desired hydro(chloro)fluoroalkane of all therecoverable intermediates; the yield of hydro(chloro)fluoroalkane is theproduct of the degree of conversion and the selectivity for thishydro(chloro)fluoroalkane.

EXAMPLES 1-7

[0032] 1.61 of an aqueous solution comprising 0.56 mol/l of nitrates ofchromium and of other metal M and exhibiting the desired atomic ratio ofthe chromium to the metal M were prepared. 0.7 l of aqueous ammoniasolution, exhibiting a concentration of NH₄OH of 4 mol/l, was added tothe solution with stirring at ambient temperature. The precipitate wascentrifuged. The cake was washed several times with water atapproximately 65° C. until at least 80% of the initial content ofammonium ions in the cake, confirmed using Nessler's reagent, had beenremoved. The washed cake was dried for 2 days at 105° C. Theagglomerates obtained after drying were milled to produce grains with asize of less than 5 mm. The grains were subjected to calcination for atotal period of 69 h while flushing with nitrogen. The temperature wasmaintained first at 215° C. and then at 330° C. The specific surface(SS) of the mixed oxide obtained is shown in Table 1. It was determinedaccording to the BET method with nitrogen, measured on a Carlo ErbaSorptomatic® 1990 device after degassing the samples at ambienttemperature for 12 h under a vacuum of 10⁻⁵ torr. The content ofammonium ions in the mixed oxide is also shown in Table 1.

[0033] The mixed oxide was introduced into a tubular reactor made ofHastelloy C. A fluorination treatment was carried out with hydrogenfluoride gas (10 g/h per 100 cm³ of mixed oxide) mixed with nitrogen for8 h at a temperature of 200 to 350° C. The specific surface (SS) of thecatalysts which are obtained is shown in Table 1.

[0034] 10 cm³ of the catalyst and a hydrogen fluoride/ perchloroethylene(PER) mixture in a molar ratio of 10 mol/mol were introduced into atubular reactor with an internal diameter of 15 mm. The reactionpressure was maintained at 1 bar and the temperature at 350° C. Theresidence time was 12.5 seconds.

[0035] The results are collated in Table 1 below. It emerges therefromthat the process according to the invention makes it possible to obtaina high yield of HFC-125. The process gives a high overall selectivity athigh degrees of conversion. TABLE 1 Over- De-gree all SS Mixed SS ofcon- sel- HFC- M/Cr mixed oxide cata- ver- ecti- 125 atomic oxide NH₄ ⁺con- lyst sion vity yield No Metal ratio (m²/g) tent (g/kg) (m²g) (%)(%) (%) 1 Al 10/90 299 0.05 155 97.5 89.8 61.0 2 Al 50/50 351 0.003 7881.1 92.4 36.7 3 Al 70/30 343 <0.002 68 89.3 94.4 48.0 4 Al 90/10 3510.004 54 79.2 94.0 35.1 5 Zr 10/90 238 <0.002 77 98.0 92.8 649 6 V 10/90254 0.007 106 95.0 91.3 61.1 7 Ca 10/10 291 <0.002 26 76.0 88.7 42.6

1—Process for the preparation of a hydro(chloro)fluoroalkane accordingto which a halogenated precursor of the hydro(chloro)fluoroalkane isreacted with hydrogen fluoride in the presence of a catalyst comprisingchromium (Cr) and at least one other metal (M) selected from aluminium,barium, bismuth, calcium, cerium, copper, iron, magnesium, strontium,vanadium and zirconium, in which process the content of ammonium ionsNH₄ ⁺ in the catalyst is at most 1% by weight. 2—process according toclaim 1, in which the content of ammonium ions in the catalyst is atmost 0.2% by weight. 3—Process according to claim 1 or 2, in which thecatalyst exhibits a specific surface, determined according to the BETmethod, of at least 15 m²/g. 4—Process according to any one of claims 1to 3, in which the catalyst was obtained by fluorination of a mixedoxide of chromium and of metal M. 5—Process according to claim 4, inwhich the mixed oxide was obtained by coprecipitation by reaction of anaqueous solution of soluble metal and chromium salts, which is reactedwith an aqueous ammonia solution. 6—Process according to claim 4 or 5,in which the mixed oxide exhibits, before fluorination, a specificsurface, determined according to the BET method, of at least 150 m²/g.7—Process according to any one of claims 1 to 6, applied to thesynthesis of a hydrofluoroalkane. 8—Process according to claim 7, inwhich the metal is selected from aluminium, calcium, cerium, vanadiumand zirconium. 9—Process according to claim 8, in which the metal isaluminium. 10—Process according to claim 9, in which the Al/Cr atomicratio is from 0.5 to
 20. 11—Process according to any one of claims 7 to10, in which the hydrofluoroalkane obtained is pentafluoroethane.12—Process according to any one of claims 1 to 6, applied to thesynthesis of a hydrochlorofluoroalkane. 13—Process according to claim12, in which the metal is selected from barium, bismuth, copper, iron,magnesium and strontium. 14—Catalyst comprising chromium (Cr) and atleast one other metal (M) selected from aluminium, barium, bismuth,calcium, cerium, copper, iron, magnesium, strontium, vanadium andzirconium, in which catalyst the content of ammonium ions NH₄ ⁺ is atmost 1% by weight. 15—Catalyst according to claim 14, in which thecontent of ammonium ions NH₄ ⁺ is at most 0.2% by weight. 16—Mixed oxidecomprising chromium (Cr) and at least one other metal selected fromaluminium, barium, bismuth, calcium, cerium, copper, iron, magnesium,strontium, vanadium and zirconium, in which mixed oxide the content ofammonium ions NH₄ ⁺ is at most 1% by weight. 17—Mixed oxide according toclaim 16, in which the content of ammonium ions NH₄ ⁺ is at most 0.2% byweight.